Regeneration of Critical Zone Structure: Macroporosity and Soil Gasses in Old-Field Forest Soils of the Southeastern US.

Macropores allow water drainage vertically between soil horizons and laterally through horizons in near-saturated conditions. Because of this, soil macroporosity can be very important for the flow of water in soils which have regions of high clay content or frequently experience high precipitation events. Soil macropores were lost and destroyed by human conversion of forests, largely hardwood oak and hickory, to agricultural plantations in the Southeastern US from the 1700’s through the early 20^th century. Loblolly pine forests have reclaimed much of this landscape and have been growing for more than fifty years in many places. We are working to quantify the degree to which macroporosity has been regenerated under these old-field forests through the action of tree roots and soil fauna relative to reference hardwood forests and continuously cultivated agricultural fields. Several metrics are being used including measurements of saturated hydraulic conductivity (K-sat), soil bulk density, macroinvertebrate abundance, soil aggregate stability, and X-ray computed tomography (CT) analyses. Direct X-ray CT quantitative measurements of macropore length, volume, and connectivity in intact soil cores paired with lab and field analyses are allowing for fine characterization of the regeneration of soil structure in these soils. Preliminary results indicate that many old-field Loblolly forest soils continue to have diminished macroporosity which has only partially regenerated. Soil gasses monitored at depth in these locations support these results with soil carbon dioxide being tightly coupled to seasonality indicative of actively respiring roots. In old-field forest plots, high concentrations of carbon dioxide from 1.5-3m decreasing gradually by 5m depth suggests that living macropore-forming root networks are present and respiring in the soil profile at several meters depth. This is not evident in the agricultural fields were the shallow annual row-crop root-zone is plowed every year.